Disturbing noise—in contrast to a useful sound signal—is sound that is not intended to meet a certain receiver, e.g., a listener's ears. The generation process of noise and disturbing sound signals can generally be divided into three sub-processes: the generation of noise by a noise source, the transmission of noise away from the noise source and the radiation of the noise signal. Suppression of noise may take place directly at the noise source, for example, by means of damping. Suppression of noise may also be achieved by inhibiting or damping the transmission and/or radiation of noise. Noise control methods and systems are increasingly utilized to eliminate or at least reduce the noise radiated into a listening room by means of destructive interference, i.e., by superposing the noise signal and an appropriately controlled compensation signal. Such systems and methods are summarized under the term active noise canceling or active noise control (ANC).
Although it is known that “points of silence” can be achieved in a listening room by superposing a compensation sound signal and the noise signal to be suppressed such that they destructively interfere, a reasonable technical implementation was not feasible until the development of cost-effective, high-performance digital signal processors, which may be used together with an adequate number of suitable sensors (microphones) and actuators (loudspeakers).
Today's systems for actively suppressing or reducing the noise level in a listening room (known as “active noise control” or “ANC” systems) generate a compensation sound signal of the same amplitude and the same frequency components as the noise signal to be suppressed, but with a phase shift of 180° with respect to the noise signal. The compensation sound signal interferes destructively with the noise signal and the noise signal is thus eliminated or dampened at least at certain desired positions within the listening room.
In the case of a motor vehicle, the term noise encompasses, inter alia, noise generated by mechanical vibrations of the fans, engine and components mechanically coupled thereto, as well as wind and tire noise. Modern motor vehicles may have such features as so-called “rear seat entertainment”, which presents high-fidelity audio using a plurality of loudspeakers arranged within the passenger compartment of the motor vehicle. In order to improve sound reproduction quality, disturbing noise can be considered in digital audio processing. Besides this, another goal of ANC is to facilitate conversations between people sitting in the rear seats and people sitting in the front seats.
Modern ANC systems depend on digital signal processing and digital filter techniques. A noise sensor (e.g., a microphone) or a non-acoustic sensor (e.g., a rotational speed sensor coupled to the engine of a motor vehicle) may be employed to obtain an electrical reference signal that represents the disturbing noise signal generated by a noise source such as an internal combustion engine of a motor vehicle. This so-called reference signal may be fed to an adaptive filter; the filtered reference signal is then (e.g., after further signal processing and amplification) supplied to one or more acoustic actuators (e.g., loudspeakers), which generate a compensation sound field in phase opposition to the noise within a defined portion of the listening room. Thus, the noise within this defined portion of the listening room can be eliminated or at least dampened. The residual noise signal may be measured by means of one or more microphones. The resulting microphone output signal(s) may be used as an “error signal” that is fed back to the adaptive filter. The filter coefficients of the adaptive filter may then be modified such that a norm (e.g., the power) of the (e.g., multi-dimensional) error signal is minimized.
A known digital signal processing method frequently used in adaptive filters is an enhancement of the known least mean squares (LMS) method for minimizing the error signal, or the power of the error signal to be precise. These enhanced LMS methods are the filtered-x LMS (FXLMS) algorithm or modified versions thereof, as well as related methods such as the filtered-error LMS (FELMS) algorithm. A model that represents the acoustic path(s) from the acoustic actuator(s) to the error signal sensor(s) (e.g., an error microphone) is used to implement the FXLMS (or any related) algorithm. This acoustic path, or paths in the multi-channel case, from the loudspeaker(s) to the error microphone(s) is usually referred to as the secondary path of the ANC system, whereas the acoustic path(s) from the noise source to the error microphone(s) is/are usually referred to as the primary path of the ANC system.
ANC systems are usually designed to achieve maximum damping throughout the spectral operational range, which is achieved by minimizing the power of the error signal using the aforementioned LMS methods. Particularly in multi-channel ANC systems, the residual power of the noise (i.e., the error signal) may vary depending on the operating point of the ANC system (e.g., on the current rotational speed of a car engine in the case of an automobile application). In automobile applications, the noise spectrum depends heavily on the rotational speed (measured in rotations per minute, or rpm) of the engine; the spectrum of the noise thus usually has a maximum at a fundamental frequency (or a related higher harmonic), which corresponds to the rotational speed of the engine. At a rotational speed of 2,400 rpm, the fundamental frequency may be, for example, 40 Hz (and 50 Hz at 3000 rpm and so on). The achievable damping (attenuation) of the noise and thus the residual power of the noise may vary depending on the fundamental frequency (i.e., the rotational speed) that may perceived as unpleasant by a listener. There is thus a need for an improved ANC system that eliminates or at least alleviates the mentioned variations of residual noise.